The present invention relates to an overvoltage protection device.
FIG. 1 shows the conventional arrangement of a protection device 1. Given a power supply voltage available between the input terminals A and B, and an electronic circuit 2 to be protected, the protection device is positioned between the input terminals A and B.
Two main types of protection devices are commonly used.
A first type of protection device, for example a zener diode, is designed to clip the overvoltage pulses occurring between terminals A and B. The current/voltage characteristic of this component is of the type illustrated in FIG. 2A, namely, as soon as the voltage across the zener diode terminals exceeds a determined value, called breakdown voltage or avalanche voltage V.sub.BR of the diode, the current increases up to a substantially constant voltage. Thus, as shown in FIG. 2B, pulses P1 and P2 that are added to a supply voltage such as a full-wave rectified voltage are clipped and, upon the end of the pulse, the normal supply is again present across the terminals of the device 2 to be protected.
A second type of protection device, such as a triggering avalanche thyristor exhibits the current/voltage characteristic shown in FIG. 3A. As soon as the voltage applied to this device exceeds a value V.sub.BO, or break over value, the device becomes conductive and the voltage across its terminals drops to a very low value. The system then remains at the conductive state as long as the supply current is not decreased to a value lower than a hold current I.sub.H. By way of example, voltage V.sub.BO can be about a few hundreds volts and the voltage V.sub.H about ten volts. The effect of such a protection system on a full wave rectified voltage is illustrated in FIG. 3B. It can be noted that, from pulse P1, the device 2 is no longer supplied until the following half-period.
Each of the protection devices above described exhibits advantages and drawbacks.
A major drawback of zener diode systems is that, when pulses have a long duration time, a substantially high current flows during the pulse duration though the diode which has a high voltage across its terminals (about 400volts, for example, for a mains protection device) which causes an increase of the diode temperature . It is then necessary to provide large size and costly diodes.
A major drawback of devices of the avalanche thyristor type is that, after each overvoltage, supply is interrupted until resetting of the supply voltage. Thereby, malfunctions occur in the device to be protected that is no longer energized or that has to include a high input tank capacitor to palliate these voltage drops. Despite this drawback, one is induced to use protection devices of this type when overvoltages are liable to be of high energy (large amplitude or duration).
However, in practice, the problem encountered is somewhat different. Indeed, FIG. 4 shows the results of a statistic survey achieved on subscribers' lines in Europe. This survey corresponds to an observation for 112 days of a subscriber's line and shows the occurrence of 1009 overvoltages. More particularly, FIG. 4 is a table showing the probability of occurrence of overvoltages of determined amplitude and duration. The table of FIG. 4 shows that 29.44% of the observed overvoltages have a value ranging from 200 to 300 volts above the normal mains voltage and a duration ranging from 1 to 3 microseconds whereas 0.42% only of the observed overvoltages have a value ranging from 600 to 700 volts and a duration ranging from 3 to 10 microseconds.
The observed overvoltages can be classified into two groups: high energy and low energy overvoltages. High energy overvoltages are characterized either by a long time duration (for example over 10 microseconds) even if their amplitude is relatively low (for example lower than 300 volts), or by a high amplitude (for example over 600 volts) even if their duration is relatively short (for example smaller than one microsecond). Low energy overvoltages exhibit complementary characteristics. In the above example, they have an amplitude lower than 600 volts and a time duration smaller than 10 microseconds.
Referring to the table of FIG. 4, it can be noted that low energy pulses occur in 96.38% of the observed cases, whereas high current overvoltages represent only 3.62% of the cases. However, conventionally, and to take into account high energy overvoltages, a clipping-type protection device such as a zener diode is not sufficient and it is necessary to resort to a shorting-type device such as avalanche thyristor.